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EdFuentetaja / EdFuentetaja.github.io

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sdr/m_m_gnu_radio_analysis/ #21

Open utterances-bot opened 6 months ago

utterances-bot commented 6 months ago

Analysis of the GNU Radio M&M Clock Recovery Implementation - Notas notables

https://edfuentetaja.github.io/sdr/m_m_gnu_radio_analysis/

Gabotero commented 6 months ago

Hi Eduardo, thank you for the article. It is full of valuable insights. There's still something I'm struggling with. Specifically, about the 8-tap FIR filter. Is it supposed to generate a new sample every 8 and place it in the middle of those 8? Is it always 8 taps no matter what the samples per symbol are?

Thanks!

EdFuentetaja commented 6 months ago

Thanks. The demodulator commented at this note runs with a sampling rate of 50 kHz. Being the AIS symbol rate = 9600 baud that means that every 5.20833 samples one symbol needs to be produced. If I remember correctly this demodulator takes 8 samples around the sample number round(5.20833*k) where k is an integer, and uses them to interpolate the value of our signal at the estimated timing _dmu. The value of 8 is the designer's choice. It could be more or less. A good idea is to try the simplest first, which will be a linear interpolation using only two samples and see if this is good enough, for many applications it should be. If there is interest I can cover the topic of timing interpolators in a future note. Thank you.